`Lucky::Params#nested_file?` returns a hash always, although its
signature says it returns a nilable hash.

Even if nested params is not empty, Lucky would still raise
`Lucky::MissingNestedParamError` if any array attributes are empty.

You had to fill in the array attribute to avoid the error:

```
response = client.exec(Features::Create, feature: {
  choices: Array(String).new, # Without this, you get "Missing param key: 'feature' (Lucky::MissingNestedParamError)..."
  maximum: 99,
  minimum: 3,
  name: "Number of Kitchens",
  type: :integer
})
```